Limited access roadways are generally built on grade spanning other roadways, railroads, waterways and other natural impediments. Roads and rail facilities also traverse over limited access roadways on overpass bridge structures. The typical overpass bridge is supported with a center pier located in the middle of the limited access roadway, the center pier supports beams that span between the end abutments. The center pier structure and beams supports the bridge deck, which facilitates transportation vehicles. Bridge beams are designed to react to dead and live loads at prescribed locations along the beam. Beams are either simply supported (support at each end of the beam), or continuously supported (supports impart a moment into the beam). The height clearance of overpass structures are standardized by statute for vehicle, types to maintain a safe minimum vertical clearance. Typical overpass bridge sections are referenced in the figures enumerated below.
Many urban environments are experiencing rapid traffic growth on limited access roadway facilities. Overpass structures generally limit the capacity to expand limited access roadways. In particular, expansion of these facilities is limited by the originally designed overpass span lengths. Moreover, most overpass bridges over roadways that need expansion are well within their useful life and have not achieved their overpass traffic capacity. Therefore, expansion of limited access roadways, beyond the original build out design, may cause the removal and reconstruction of overpass bridges before the end of their useful life. Other factors also limit the expansion of limited access roadways such as right-of-way cost, budget restrictions and environmental impacts.
Transportation agencies are seeking alternative forms of transportation that mitigate cost and environmental impacts, while providing additional capacity and enhancing commuter travel time.